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Brake pedal at half-mast

Discussion in 'E28 M5 (1988)' started by robertusmax, Aug 15, 2009.

    robertusmax guest

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    Saturday, August 15, 2009

    I have a 1988 e28 M5, which I have owned since new. I am now "racing" the car in DEC events. I hung rear brakes from a 1994 540i in June 2003 (that was ok, with no deterioration in the brake pedal at that time) and then Nurburgring brakes in the front from an European spec 1995 e34 touring M5 in May 2004. Ouch! Since adding the Nurburgring in 2005 I have had a "soft but effective" pedal. I have attempted to fix the pedal by bleeding, re-bleeding, steel brake lines, rebuilding the calipers (all 4), new pads, new/resurfacing the rotors, and replacing the pressure accumulator (the bomb). Most recently I put on a new Master cylinder for the 1993 730i on the undocumented inference that its stroke volume is greater than stock (the bore diameter is 25 mm rather than 23 mm, an increase of 18% in volume if the stroke lengths are the same). This effected no change. This week I had the dealer check the hydraulic pressure distributor switch as well as the pressure accumulator, re-bleed, check the lines, check all 4 calipers... I have not touched the Hydraulic brake booster which looks like a M/C and to which the M/C bolts, and which stands between the Power Steering Fluid and the Brake Fluid nor the brake pressure hydraulic switch... My dealer mechanic tells me they are functioning properly. I now have an intractable problem of the brake pedal feeling non-existent or soft halfway to the floor (or more), and then firm brake pedal... but oh, how heart-stopping is that 1/2 second delay till effective braking... both on the road and at the track.
    Our logical conclusion is that the stroke volume from the master cylinder is too low. We think the original brakes required less volume to move the pistons than do the 540i rear b rakes + the euro spec e34 front brakes. Although intellectually appealing, it may still be untrue, however.
    a; Is this inference true? Data?
    b: If the Stroke Volume inference were true, does it make any difference?
    c: Where else might the fault be? I can not safely drive the car in this state.
    d: How can I fix it?
    Robert M. Shuman
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    CRKrieger

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    I think it's both true and that it makes a difference. The physical facts and the hydraulic calculations tell you that. I can tell you that, in an extreme case I saw ~20 years ago, a stock E30 master could not move enough fluid to get the pads to engage on the first pump of the pedal! Worse, on the second pump, the brakes would lock up easily because the ABS could not release enough fluid! So it does not surprise me to see what you're experiencing.
    I don't think the fault lies elsewhere. To fix it, you're going to have to do some hydraulics math. First, figure out how big the combined cross-sectional area of all your original wheel cylinders was. Then, find out how big your current ones are. The difference is approximately how much bigger a master cylinder you will need. I'm betting you maybe added 50% more piston area in there and you'll need a lots bigger master to handle it.

    robertusmax guest

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    Monday, August 17, 2009
    Dear CR Krieger;
    OK, here is the math:
    1. Displacement volume of the wheel cylinder Brake Piston(s):
    1/2(D) times (pi) times (stroke*) = cc where pi is constant and stroke is constant. *I have assumed that the "stroke" of the piston is constant, since it represents the lateral displacement of the wheel cylinder piston as that piston moves from its resting position in the wheel cylinder to its braking position applied against the surface of the rotor. Constants can be dropped in this discussion about the change in volume.
    Diameter Surface Area Increased Vol Original e28M5 Rear Calipers (1 piston): 35mm 306mm^2 94cc,
    540i Rear Piston Calipers (1 piston): 40mm 400mm^2 31%

    Original e28M5 Front Calipers (4 pistons): 40mm 400mm^2x4=1600cc
    Euro Spec e34 M5 Calipers (4 pistons, (400x2) + 168cc,
    but 2 at 40 mm and 2 at 44mm): (484x2) = 1768cc 11%

    2. In terms of the different master cylinders failing to cure the problem, here are their data (sort of, since I do not know the stroke length) (again), and I am again assuming that stroke length is constant between the two cylinders:
    Diameter Volume Increased Vol.
    Original e28 M5 M/C: 23mm 132.25cc 24cc,
    New 1991 750 M/C: 25mm 156.25cc 18%

    Hmmm, now that I have done that, I am still not sure. The absolute change in required volumes may be more important that the percentage change in required volume delivery! I can certainly envision a situation where the relatively small 11% increase required by the e34's pistons is beside the point compared to the absolute increase of 168cc. I really do not have any idea of how much extra capacity is built into the braking systems upstream of the wheel cylinders, but this is only (94cc + 168cc)/ (306cc +1600cc) =262cc/ 1906cc, or 14% increased demand in the volume of Brake Fluid... Again, thanks CR for the thought, but now I have to interpret the results. Ideas? Anybody???
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    CRKrieger

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    I almost hate to do this now but ... front-rear proportioning could also have a significant effect. You might be 'overbraked' in the rear and 'underbraked' in the more critical front. Of course, if it's a 'track rat', an adjustable proportioning valve should be no sweat to add.

    robertusmax guest

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    Monday Sept. 14, 2009
    RE: Brake Pedal at half-mast

    I think I have the solution(s). I bought a new Hydraulic Brake Booster from BMP. I installed this, and obtained perhaps 35-50% improvement in the pedal. I mean, there was no longer that heart-stopping lack of response until the pedal was depressed to its effect-point (perhaps as much as 3 inches?). There was now the return of a graded effect from the top of the pedal to the hard-point of braking control. My heart no longer fibrillated in fear and anxiety before the hard brake effectiveness point. BUT. It was not what I wanted. I wanted the "hard point of braking control" within less than an inch of the top of the pedal, and NOT a graded effect with a falling pedal until I hit that hard point. So my mechanic said (and actually had said to me 2 weeks before I gave him the problem) "You do not suppose the brake pedal push rod for the Hydraulic brake booster is miss-adjusted?" No, said I. Humph. He was right. I brought back the car from my test drive with the new hydraulic brake booster. I wanted compliment him on the improvement with the booster. And I wanted to complain that the improvement was still only 50%, and still was not right. So he grimaced, talked about bleeding the ABS, and then spent the next day adjusting the booster's brake pedal push rod. Voila! He was right, he was right, he was right. Brake pedal up and tight. Brakes effective, now! Hooray for all my modifications with big brakes and a fast car.
    Lessons learned? The braking system is very complicated. Everything is interconnected. Component parts can fail together (the Hydraulic Booster can fail progressively AND the pedal height adjustment can be wrong). Simple fixes like a mechanical adjustment can be overlooked. If the booster replacement and then the adjustment of the pedal pusher had not worked, bleeding the ABS was next on my agenda. Persistence pays. Thank heavens for gifted mechanics.

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